This fixes the cosmopolitan platform.
- Switch `build_cosmocc.sh` and platform documentation to
explicitly use the x86_64 cosmocc compiler as multi-arch
cosmocc won't work here. Older version `cosmocc` just did
a x86_64 build.
- Add missing items from `platform_internal.h` to fix build.
Fixes the Cosmopolitan Libc platform attempting to use `/dev/urandom`
on operating systems that do not have it.
Signed-off-by: G4Vi <gavin@dylibso.com>